Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 63

Superconducting solenoids are attractive because of their lack of an external energy source, high stored magnetic field energy, and temporal stability, although with the requirement for a cryogenic subsystem. Permanent magnets offer these capabilities to some degree and do not need cryogenics. Their downside is a reduced field-generating capability; human-sized homogeneous magnets above 0.5 T require a considerable weight of material. They are also less stable, for example drifting with temperature.

Rare-earth magnets made from alloys of neodymium such as NdFeB are the strongest form of permanent magnets readily available. They are a relatively recent development, introduced in 1984 [104,105]. Formed into blocks from sintered powder and then magnetized by applying a pulsed magnetic field, the strength of the permanent magnet is measured by its remanence (B_r), which measures the magnetic field of the residual magnetization present in the absence of an external field. For NdFeB, B_r ranges from 1.0 to 1.4 T. The field of NdFeB changes with temperature, with a temperature coefficient of remanence of about −0.1% per °K (the field goes down as the temperature increases). The coercivity (H_cl) measures the material’s resistance to demagnetization by an external field (or the field of a neighboring block). The tendency of the sintered material to corrode requires that the block be coated, usually with copper-nickel plating, or a coating of epoxy or another polymer. Because of the considerable forces between large NdFeB blocks, handling and assembling the material requires care and expertise.

The most common geometry for MRI use is a simple two-pole-piece dipole magnet. Spacing two magnetized disks with a gap (containing the imaging region) forms a uniform dipole field volume between the pole pieces. Each disk creates a dipole current pattern, and the net effect is like two stacked current loops (each with the current flowing in the same sense). Optimizing the placement of the material in the disks and/or shaping an iron pole-piece can improve homogeneity. Adding an iron yoke helps channel the flux into a closed circuit and can provide mechanical support for the two disks (which otherwise want to come together). This is the basic configuration of many commercial 0.2–0.35-T “low-field open” MRI systems, which reached a peak in popularity in the early 1990s. The “open” patient space reduced claustrophobia anxieties at a time when superconducting solenoid systems were quite long and narrow bore (55 cm diameter as opposed to the current norm of 70 cm diameter). In many ways, low-field open systems remain a reasonable choice for emergency MR, but their size and weight are comparable with 1.5-T superconducting systems, requiring a similar siting footprint. The Hyperfine 64-mT portable MRI system [36] appears to follow this geometry, as does a 0.2-T portable system mounted in a mini-van for elbow imaging at baseball games [16]. The direction of the B₀ field differs from their superconducting cousins. The superconducting solenoid imposes B₀ in the “head–foot” direction, while low-field open systems usually have a posterior to anterior (vertical) B₀. It has been noted that the accompanying change in radiofrequency field geometry for low-field open geometry systems yields a reduced interaction with deep-brain stimulation leads (and lower radiofrequency heating) [106].